1. Field of the Invention
The present invention relates to novel molds designed for the manufacture of articles of expanded, non-polar plastics material, by means of ultra-high-frequency radiation (UHF).
2. Description of the Prior Art
UHF waves, which are currently named microwaves and which are situated between about 300 MHZ and 30 GHZ, are very widely applied at the present time, both in the medical field and in the scientific or industrial field.
This energy is instantaneously available, it is propagated in all atmospheres (under vacuum, under pressure, in a cold, hot or ventilated environment, . . . ), and it can also be very easily servocoupled. The principle thereof is very simple: the application of a continuous electrical field to an electrically sensitive material aligns the molecules of the said material parallel to the field as if it involved micro-magnets. In an electrical field alternating at very high frequency, the magnet-molecules cannot follow the lines of the fields due to inertial forces. It is hence by knocking these inertial forces that "microwave" energy is transformed into heat. On the other hand, when the material treated is electrically neutral, the microwaves pass through it, as if it did not exist: it is said that the material is transparent. Accordingly, attempts have been made to utilize this particularly economical and highly selective energy--enabling a specific effect on a given target--in the field of manufacturing expanded plastics materials. Thus, by moistening a non-polar plastics material (hence transparent to UHF radiation) in the form of preexpanded particles of, e.g., polystyrene containing a blowing agent, with a polar liquid, and by subjecting these particles so moistened to UHF radiation, expanded plastics materials are produced elegantly and economically (cf. particularly commonly assigned French Patent Applications Nos. 2 298 571 and 2 368 513). Such a technique constitutes a considerable advance with respect to traditional transforming techniques, consisting in introducing into a mold a certain amount of unexpanded material and then causing pressurized steam to penetrate therein, at a sufficient temperature to contribute the heat necessary to initiate the expansion process.
However, this method of molding by means of microwaves still has a drawback: the molds used are conventional molds, made of a material transparent to UHF radiation (or absorbing very little of this radiation), which has as a consequence an absence of homogeneity of heating and an increase in the expenditure of energy, due to the fact it is necessary to reheat the walls of the mold on each molding cycle. This drawback has been surmountable by means of a novel device developed by my assignee (cf. French Patent Application No. 2 402 526), which device comprises a mold whose impression is formed of a material having high dielectric losses, absorbing UHF radiation, whilst the body itself of said mold is formed of a material non-absorbent to UHF radiation.
These particularly advantageous methods and devices, requiring no capacitative circuit, enabling the use of voltages which are not high and ensuring a high homogeneity of the molded, expanded plastics materials obtained, have assuredly been at the origin of the very considerable development of foamed plastics. However, it has been observed that the necessity of having cavities possessing very precise electrical characteristics, made it a requisite to incorporate in said cavities additives possessing loss factors which were linear with temperature. These additives (for example carbon, titanium oxide, barium titanate, etc. . . . ) are often expensive, the preparation of said cavities is always difficult, and moreover, the mechanical characteristics of said cavities sometimes leaves something to be desired. Accordingly, novel molds have been proposed with a low or zero loss angle, and with a cavity which is produced in porous material. The porous nature of this cavity results in the situation that, if the said cavity is transparent to UHF radiation in the dry state, on the contrary it absorbs UHF radiation on the addition of a polar liquid, since this liquid is absorbed by the pores of said impression (cf. in particular commonly assigned French Patent Application No. 2 441 474 as well as French Patent No. 1,508,963 of R. B. BUONAIUTO). Porous molds are simple in design and do not necessitate any additive absorbing UHF radiation.
However, after several years of operation the following phenomenon has been observed: the heat gradient in the porous mass (for example, the ceramic wall) being reversed with respect to conventional heating, it results therefrom that the pressure of the steam (or of another polar substance) formed by the conversion of electromagnetic energy into heat is greater in the interior than on the surface of said porous wall. As a result, at the beginning of heating, whilst vaporization has already started in the interior, the surface layers are still in a liquid phase. The pressure in the interior of the porous mass being higher than at the surface,--and since the porous structures of all cavities and walls used until now are monolithic--, this vapor pressure coming from the inside of the porous mass, prevents the balanced reabsorption of the water occurring at the surface or condensed on the surface of the wall or of the cavity, in contact with the foamed plastics. In other words, the capillary pressure of all the cavities and porous walls of molds used until now being lower than the pressure of the steam (or of another polar substance) coming from the inside of the porous mass, the recycling and reabsorption of the water (or of another polar substance) condensed on the surfaces of the walls and in contact with the foamed plastics is thus considerably disturbed. To avoid this drawback, it is at present necessary either to stop the heating from time to time to permit suitable reabsorption of the condensed water, or to introduce additional heating (as is the case in French Patent No. 1,508,963) to evaporate completely the condensed water. In both cases, there is an undesirable increase in the cost price of molding, which can reach proportions which are not negligible.
It is a particular object of the present invention to overcome these disadvantages by providing a novel mold for the fabrication of expanded plastics materials in particulate form (e.g., polystyrene beads) which responds to the various necessities of practice better than previously known molds with a porous cavity (such as for example those described in French Patent Application No. 2 441 474 and French Pat. No. 1 508 963).
It is also a particular object of the invention to provide a novel mold and method of molding which permit a substantial economy in energy and which permit a considerable shortening of the fabrication cycle.
Other objects and advantages of the present invention will become apparent from the description which follows.